IF I have flown the route literally 100+ times over 30 years, I can say that I have maybe once had a ticket "in hand" 4 months out. My average would be probably be less than 2 weeks out, although I sometimes have them a month out, and I would say that mine would be liberal pattern on the route, that has lots of finance types often doing so less than 48 hours out (and who often have corporate net rates as well), Ideally the route with BA/AA is supposed to operate once per hour 16 hours a day......BTW you do realize that traffic from the NYC area to the London area is in actual fact close to twice that, right? There is no comparison between a long thin route like SEA-LHR, that maybe in its history has seen 2x flights per day, and NYC-LON, let alone JFK-LHR.